[Leetcode] Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

 

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

 

Solution 1: 非递归

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public List<Integer> preorderTraversal(TreeNode root) {
12         List<Integer> result=new ArrayList<Integer>();   
13         if(root==null){
14             return result;
15         }
16         Stack<TreeNode> s=new Stack<TreeNode>();
17         result.add(root.val);
18         s.add(root);
19         TreeNode p=root.left;
20         
21         while(!s.isEmpty()){
22             while(p!=null){
23                 s.add(p);
24                 result.add(p.val);
25                 p=p.left;
26             }
27             TreeNode n=s.pop();
28             p=n.right;
29             if(p!=null){
30                 s.add(p);
31                 result.add(p.val);
32                 p=p.left;
33             }
34         }
35         return result;
36     }
37 }

 

Solution 2: 递归

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public List<Integer> preorderTraversal(TreeNode root) {
12         List<Integer> result=new ArrayList<Integer>();
13         myPreorderTraversal(root,result);
14         return result;
15     }
16 
17     private void myPreorderTraversal(TreeNode root, List<Integer> result) {
18         // TODO Auto-generated method stub
19         if(root!=null){
20             result.add(root.val);
21             myPreorderTraversal(root.left, result);
22             myPreorderTraversal(root.right, result);
23         }
24     }
25 }

 

posted @ 2014-10-22 13:36  Phoebe815  阅读(144)  评论(0)  编辑  收藏  举报